Generally, high purity is required for polycrystalline silicon used as a raw material for producing a semiconductor and the like. It is therefore required that the chlorosilanes used as a raw material for producing polycrystalline silicon should have very high purity. For example, when boron and phosphorus are contained in chlorosilanes as impurities, even a very small amount of these impurities will significantly affect the electrical properties (resistivity) of polycrystalline silicon. Therefore, it will be practically meaningful to provide a technique of efficiently removing boron impurities and phosphorus impurities contained in chlorosilanes.
Generally, crude chlorosilanes are obtained by a known method from a metallurgical-grade silicon (so-called metal-grade silicon, hereinafter referred to as “metallic silicon”) which contains a relatively large amount of impurities, and the crude chlorosilanes thus obtained are further purified to high purity by distillation or the like to obtain chlorosilanes. However, since boron and phosphorus are generally contained in metallic silicon in an amount in the order of several hundred ppb to several hundred ppms in terms of the element, these impurities are removed insufficiently in the process of purifying crude chlorosilanes, and boron and phosphorus may remain as impurities in the chlorosilanes finally obtained. These residual impurities may pose a problem.
In order to obtain crude chlorosilanes, there is well known a method in which metallic silicon is brought into contact with hydrogen chloride in the presence of a catalyst to chlorinate the metallic silicon and the resulting product is distilled (for example, refer to Japanese Patent Laid-Open No. 2005-67979 (Patent Literature 1)). The crude chlorosilanes refer to a fraction in this distillation and are generally a mixture of one or two or more chlorosilanes selected from dichlorosilane, trichlorosilane, and tetrachlorosilane.
Boron impurities and phosphorus impurities contained in metallic silicon are chlorinated at the same time as the crude chlorosilanes are produced, and are mixed into the crude chlorosilanes as a form of compounds having various structures. Such crude chlorosilanes are purified to obtain chlorosilanes, but it is difficult to separate and remove, in the distillation step, compounds having boiling points which are close to those of the chlorosilanes to be finally obtained. Boron compounds and phosphorus compounds may be thus mixed into (may remain in) the distillation fraction as impurities. If polycrystalline silicon is produced using such chlorosilanes, boron and phosphorus will be incorporated into the polycrystalline silicon, leading to a result that cannot obtain a polycrystalline silicon having desired properties.
The main reason why it is difficult to remove boron and phosphorus as impurities which are contained in crude chlorosilanes by a general distillation method is that these impurities are present in the form of compounds having low boiling points. Specifically, although the boron and phosphorus in crude chlorosilanes can take the form of various hydrides or chlorides, boron and phosphorus are usually present in the form of boron trichloride (BCl3) and phosphorus trichloride (PCl3), respectively, having low boiling points. However, a general distillation method cannot easily remove such volatile compounds from the chlorosilanes to a very low concentration level required for polycrystalline silicon to have desired properties.
Under the circumstances, various methods have been proposed as a method for reducing the content of boron impurities and phosphorus impurities in crude chlorosilanes or chlorosilanes (method for purifying chlorosilanes). For example, National Publication of International Patent Application No. 1983-500895 to D. R. D et al. (Patent Literature 2) has proposed a method comprising: introducing a small amount of oxygen into chlorosilanes at a high temperature condition to allow the chlorosilanes to react with the oxygen to form a complex; allowing the complex to react with boron impurities and phosphorus impurities to produce a new complex; and separating the new complex in the distillation step of the chlorosilanes to obtain chlorosilanes having a low impurity concentration.
Further, U.S. Pat. No. 3,126,248 to F. A. Pohl et al. (Patent Literature 3) has proposed a method comprising: producing an adduct of an organic substance containing an element having a lone electron pair such as benzaldehyde and valerolactone with boron impurities; and then distilling the resulting mixture to remove the impurity.
Furthermore, U.S. Pat. No. 3,252,752 to the same inventors (Patent Literature 4) has reported a method comprising: catching and removing boron impurities with benzaldehyde, propionitrile, or the like immobilized on an adsorbent such as activated carbon and silica gel.
On the other hand, Japanese Patent Laid-Open No. 2009-62213 (Patent Literature 5) has reported that when chlorosilanes containing boron impurities and phosphorus impurities are treated by introducing oxygen in the presence of an aromatic aldehyde, the boron impurities and the phosphorus impurities can be converted into high-boiling compounds at the same time and can be easily removed during the distillation of chlorosilanes, without using a high temperature as in a method disclosed in Patent Literature 2.
The method for removing impurities using an aromatic aldehyde as disclosed in Patent Literature 5 allows the impurities to be converted into high-boiling compounds, the distillation of the impurities from the bottom of a single evaporator or a distillation column to be suppressed, and the impurities to be concentrated and removed at a high concentration, thus allowing reduction in the amount of trichlorosilanes which accompanies the disposal. Therefore, this method also has advantages in cost.
However, as a result of investigation by the present inventors, it has been found that when a large amount of chlorosilanes is treated with an aromatic aldehyde, solids derived from the aldehyde compound may be newly produced in piping or in a strainer, and the solids may interfere with the control of the purification system including a purification apparatus.
Particularly, in a place where a solution containing concentrated aromatic aldehyde stagnates such as piping for discharging a residue from a still, solid production becomes remarkable to block the piping or the strainer, which will require disassembling of the piping to perform solids-removing operation depending on the case. Further, if the concentration of the aldehyde compound in the residue is somewhat reduced by reducing the concentration rate in order to relieve the above-described bad influence of solids, the discarded amount of chlorosilane which is the target compound for purification will be increased. Therefore, the reduction in the concentration rate is not practical.